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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 23rd Jan 2022, 5:32 pm   #21
wireless_john
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Default Re: Grant Searle's retro "Simple Z80" 32k enhancement

Actually now I've played with it a bit I notice the following.

Initially, the RAM was a 62256 (32k x 8) and it works fine, reporting something just below 32k of free RAM. Then I discovered that I had a Dallas DS1225 in my box. Had it for years, probably at least 20! Its an 8k x 8 NVRAM. So I thought I'll give it a try, and yes, still working. Retains programs while power is off. They're still there after a Warm start, and a Cold start erases them.

However, when booted up and a cold start is requested, it still reports just under 32k of RAM! so does PRINT FRE(0).

If I examine the non-existent RAM above 8k, contents are reported as HEX 7E.

Not quite sure what's going on here.

Also, CLS doesn't clear the screen. Maybe that's something to do with the VT100 emulation. That is to say, perhaps it wouldn't do it on a real VT100 either.

Any ideas?

John
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Last edited by wireless_john; 23rd Jan 2022 at 5:34 pm. Reason: typos
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Old 23rd Jan 2022, 7:18 pm   #22
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Default Re: Grant Searle's retro "Simple Z80" 32k enhancement

Hi John, I think the address decoding is repeating the 8k ram four times in the 32k space and the memory test is not recognizing that they are shodows of the same memory. If you enter a program greater than 8k you might find it corrupts the beginning of memory and crashes basic.

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Old 23rd Jan 2022, 7:30 pm   #23
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Default Re: Grant Searle's retro "Simple Z80" 32k enhancement

Indeed - 'reflections' as they were called. Its ignoring the top two address lines.
Take the stack for example. The Z80 thinks its up just below 0xFFFF when in fact the memory chip sees the stack writing to 0x9FFF, which the Z80 thinks is part of the 32k program storage area
This is all as expected John, a consequence if an 8k ram in a 32k slot, its not a fault

By the way, you can easily mod your 62256 to NVRAM by adding a DS1210 or Marks MAX818 - link
Cheers
Phil
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Old 23rd Jan 2022, 8:22 pm   #24
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Default Re: Grant Searle's retro "Simple Z80" 32k enhancement

Thanks very much for that. Now I know!

John
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Old 24th Jan 2022, 5:27 pm   #25
Phil__G
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Default Re: Grant Searle's retro "Simple Z80" 32k enhancement

If you do a cold start, and answer "Memory top?" with 40959 you will have a fully working 8k machine with no 'reflection' problems John. Subsequent 'warm starts' will retain this 8k limit

Quote:
Originally Posted by wireless_john View Post
if I examine the non-existent RAM above 8k, contents are reported as HEX 7E.
Directly above 8k at hex address 2000h to 7FFFh is an unoccupied 24kB hole in the memory map. On the 32k version, RAM starts at 8000h. Your 7E's are empty space
With the 8k RAM installed, if you dump 8000h, A000h, C000h, and E000h you'll see the contents of the keyboard buffer repeated, and at 8200 ish, A200 ish, C200 ish and E200 ish you'll see your basic program.
Even after the Memory top=40959, these reflections still physically occur but BASIC wont use them
Cheers
Phil

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Old 25th Jan 2022, 6:57 pm   #26
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Default Re: Grant Searle's retro "Simple Z80" 32k enhancement

Thanks Phil, I'll try that.

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Old 22nd Jun 2023, 10:15 pm   #27
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Default Re: Grant Searle's retro "Simple Z80" 32k enhancement

Thread re-opened for updates by OP

Cheers

Mike T
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Old 22nd Jun 2023, 10:39 pm   #28
Phil__G
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Default Re: Grant Searle's retro "Simple Z80" 32k enhancement

Thanks Mike

Just to say I've done a small update to the BASIC/monitor rom images to include editing of command lines and to accept upper or lower case.
They're minimal changes since there are only 570 bytes to work with,
remember the monitor occupies what was the small unused space at the top of the 8k BASIC rom

Everything including compete 8k ROM images, updated source code & amended documentation is in one zip file on the Retro page of http://philg.uk

The new ROMs suit the Grant Searle Z80 and the RC2014 Mini and Micro in either 32k or 56k RAM versions. It has Spencers approval for the RC's

Cheers
Phil
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